(1) Field of the Invention
The invention relates to a method to control a gas discharge lamp, and more particularly, to a method and a circuit to digitally control the illumination intensity of a gas discharge lamp.
(2) Description of the Prior Art
Gas discharge lamps are used in a wide variety of applications. A typical example of a gas discharge lamp is a fluorescent lamp. In a gas discharge lamp, a large voltage is used to ionize the gas inside the lamp tube. Once an ionization arc has been established, continued application of electrical power causes the lamp to provide light.
It is often difficult to dim the illumination intensity of a gas discharge lamp because it is difficult to maintain a perceptibly constant arc at low illumination levels. Interruptions in the current arc cause the lamp to flicker.
Prior art solutions to controlling, and more particularly, providing dimming control of gas discharge lamps, typically involve rather complex analog circuits. Typically, an analog voltage controlled oscillator (VCO) is used to create a variable lamp driver frequency. As the driver frequency is reduced, the lamp dims. In this scheme, the VCO may drive a pulse width modulated (PWM) output to the lamp. When such analog solutions are up integrated onto an IC, they suffer from a high I/O pin count, poor noise immunity, and large silicon area. Finally, the VCO approach circuit is adversely affected by the presence of other oscillators on the integrated circuit device.
Several prior art inventions describe methods and apparatus to control fluorescent lamps. U.S. Pat. No. 6,150,772 to Crane describes a control circuit for a gas discharge lamp. A microcontroller is used to set analog voltage, current, and pulse width modulated (PWM) outputs based on a memory lookup table. U.S. Pat. No. 6,043,611 to Gradzki et al teaches a compact fluorescent lamp capable of dimming. A triac dimmer with a RC snubber is used to control illumination intensity. U.S. Pat. No. 5,204,587 to Mortimer et al discloses a fluorescent lamp control circuit that reduces the external power level to the lamp to achieve dimming. U.S. Pat. No. 6,198,417 to Paul teaches a pipelined, oversampling A/D converter using a delta sigma (xcex94xcexa3) modulator.
A principal object of the present invention is to provide an effective and very manufacturable method and circuit for controlling a gas discharge lamp.
A further object of the present invention is to provide a method and a circuit for controlling the illumination intensity of a gas discharge lamp, such as a fluorescent lamp, by modulating the oscillation frequency.
A still further object of the present invention is to provide a method and a circuit for controlling the illumination intensity of a gas discharge lamp while eliminating flicker by smoothing frequency steps using a digital delta sigma (xcex94xcexa3) modulator.
In accordance with the objects of this invention, a method to control the illumination intensity of a gas discharge lamp is achieved. The method comprises, first, converting an analog lamp illumination signal into a digital lamp illumination signal. The analog lamp illumination signal is a function of the illumination intensity of a gas discharge lamp. Second, digital target signal is subtracted from the digital lamp illumination signal to create a digital error signal. Third, a digital frequency set point is adjusted from a current value to a new value based on the digital error signal. The digital frequency set point is a high resolution digital value. Fourth, the current value and the new value are averaged by a digital delta sigma modulator to create a smoothed frequency set point. The smoothed frequency set point is a medium resolution value. Finally, an oscillating voltage signal is generated with a drive frequency based on the smoothed frequency set point. The drive frequency determines the illumination intensity of the gas discharge lamp.
Also in accordance with the objects of this invention, a circuit for controlling the illumination intensity of a gas discharge lamp is achieved. The circuit comprises, first, an analog-to-digital converter to convert an analog lamp illumination signal into a digital lamp illumination signal. The analog lamp illumination signal is a function of the illumination intensity of a gas discharge lamp. Second, a means of subtracting a digital target illumination signal from the digital lamp illumination signal to create a digital error signal is included. Third, a digital regulator circuit is used for adjusting a digital frequency set point from a current value to a new value based on the digital error signal. The digital frequency set point is a high resolution digital value. Fourth, a digital delta sigma modulator is used for averaging the current value and the new value to create a smoothed frequency set point. The smoothed frequency set point is a medium resolution value. Finally, a digital controlled oscillator is used for generating an oscillating voltage signal with a drive frequency based on the smoothed frequency set point. The drive frequency determines the illumination intensity of the gas discharge lamp.